1. Field of the Invention
This invention relates to a display unit with an LED matrix, in particular for outdoors, with display points arranged in lines and columns, which can be triggered by a control device and which are monitored for display errors by a monitoring device.
2. Description of Prior Art
It is assumed that a display unit with an LED matrix with a monitoring device for display errors is known. There, the monitoring device is elaborate and not capable to detect the exact location of an erroneous display.
A monitoring unit for display errors in connection with an LED display device in the form of a display with seven segments is disclosed in German Patent Publication DE 27 27 800, which has a current sensor transistor. The use of such a monitoring device in connection with an LED matrix does not recommend itself since, because of the plurality of display points, a large outlay would be required. A similar monitoring device for an LED display device in connection with a segment display is also disclosed in German Patent Publication DE 36 20 584 A1, wherein the open collector output of the segment display is uscd. This type of monitoring device also is not suitable for a display unit with an LED matrix.
A matrix display device is known from German Patent Publication DE 40 24 499 C1, wherein the display points are formed by means of the light output ends of optical fibers. Bistable closing elements are arranged over the light output ends of the optical fibers for creating various displays. For opening, or respectively for closing the closing elements, setting and resetting pulses are applied to the letter in lines and columns and are monitored by respective optical couplers. A separate monitoring circuit is required to determine whether the display points are actually illuminated.
It is one object of this invention to provide a display device with an LED matrix, which offers dependable functional monitoring along with a simple construction.
This object is attained as discussed in the specification and required by the claims. Accordingly, the monitoring device has a current monitoring device with a current sensor, by means of which a trigger current for each display point can be detected and assigned to the respective display point by the monitoring device. With these measures, every display point of the display device is monitored and a localization of the defective display point is available.
The outlay for triggering the display device and the required output, along with simultaneous dependable functional monitoring of the individual display points, is reduced because one current sensor is provided per line or per column. The columns and lines corresponding to the coordinate points of the display points to be activated can be triggered cyclically in multiplex operation with a frequency which results in an activation of the LEDs to be triggered above the flicker fusion frequency of the eye. In the process an optimal display quality is maintained, wherein it is possible to achieve a high luminance by the level of the triggering pulses, which can be tuned to the existing exterior light conditions by means of a modulation of the pulse width.
The location of the respectively activated display point can be simply determined because current signals of the current sensors in the control device returned by an acknowledgment line can be evaluated in respect to an error of the display point to be activated on the basis of the time information of the line triggering present in the control device, together with the time information of the column triggering. By the step the current sensors are constituted by an optical coupler, which is connected in parallel with the line triggering lines, or respectively the column triggering lines, and it is possible to detect the triggering current free of potential and to return the information regarding this to the control device for evaluation without triggering being affected.
Different evaluations and reactions in case of an error are made possible in a simple manner because the control device has an error memory in which error reports can be deposited. An error indication and/or switch-off of the display device can be controlled on the basis of the deposited error reports. For example, it is possible to make the error criteria a function of several successive measurements. It can be decided whether an error report should be made when one or several illuminated points fail, or whether, with a defined, predeterminable number of missing illuminated points, the display device should be switched off because of displays which can no longer be clearly recognized (garbling).
In order to make possible a large-area display with dependable operation, the display can be composed of several display modules, that display module can have a control unit with an error memory, which is part of the control device. Each display module is controllable and the total display can be put together from the displays of the individual display modules by means of the control device. It is also possible with these measures to determine whether a display module needs to be replaced.
The monitoring device can be completed because a reference element per display module is provided, by which a long-term change of the display properties of the display module can be detected, and/or because a temperature sensor per display module is provided, and that by means of the control unit a reference element signal, or respectively a temperature sensor signal, can be evaluated in respect to a deviation from a set value.
For example, if the achievable luminance is reduced below a predeterminable threshold, or if the color values of a color display device change by a predeterminable value in relation to a reference value, it is possible to output corresponding information for replacing the display module. It is, for example, possible to switch on a ventilator on the basis of the signal from the temperature sensor.